1. Field of the Invention
The present invention relates to a mask (X-ray mask) employed in X-ray lithography and more particularly to an X-ray mask making it possible to form a minute pattern having a high contrast that is suitable especially for manufacture of various semiconductor devices having a high density of integrated circuits.
2. Description of the Prior Art
As is generally known, X-ray lithography is a technique to replicate a pattern onto an object to be processed by selectively irradiating the object with X-rays through an X-ray mask.
The X-ray mask generally has a cross-sectional structure as shown in FIG. 1 of the accompanying drawings and includes a membrane 2 having a high permeability to X-rays that is secured by a ringshaped support frame 1 (usually made of Si), and a mask pattern 3 that is formed on the membrane 2 and made of a material having a low permeability to X-rays; i.e., a high linear absorption coefficient.
An object to be processed is placed on one side of the X-ray mask, and X-rays are applied from the other side. Thereby, a pattern having a shape corresponding to the mask pattern 3 is replicated; i.e., duplicated, onto the object.
To form the membrane 2 of the X-ray mask, it is possible to employ a large variety of organic or inorganic materials; e.g., polyimide, Si, BN, Si.sub.3 N.sub.4 or SiO.sub.2. In addition, it is possible to employ a single-layer or laminated membrane made of one or more of these materials.
On the other hand, as the material for the mask pattern 3, Au (gold) is generally employed.
The reasons for employing gold are that gold has a higher linear absorption coefficient (a low permeability) with respect to X-rays, that gold is chemically stable and exhibits a long life and that it is easy to form a film from gold by vacuum deposition or plating.
Since gold is expensive, however, the X-ray mask employing this metal is inevitably expensive, which constitutes one of serious obstacles to a wider application of the X-ray lithography in the manufacture of semiconductor devices.